Abrasive composition and tools for stone materials and ceramics

ABSTRACT

Abrasive composition for the manufacture of tools for the execution of surface treatments on stone and ceramic materials, comprising a superabrasive and a binder comprising a frit approximately melting between 450° C. and 650° C. and a refractory material. The composition also comprises a chemically inert abrasive material for thermally and mechanically protecting the superabrasive.

REFERENCE TO RELATED APPLICATIONS

The present application is the national stage under 35 U.S.C. 371 ofinternational application PCT/IT00/00354, filed Sep. 7, 2000 whichdesignated the United States, and which international application waspublished under PCT Article 21(2) in the English language.

DESCRIPTION

1. Field of the Invention

The present invention relates to an abrasive composition for themanufacture of tools for the execution of treatments of surfaces in thefield of the stone materials and the ceramics.

The invention also relates to an abrasive tool manufactured by using theabove-mentioned composition.

2. Background art

Various kinds of abrasive tools are known which have different structureas a function of the kind of use they are destined to: surfacetreatment, cut, perforation and the like. In the field of surfacetreatments the most common operations are roughing, smoothing,polishing, super finishing and the tools aimed at these operationsgenerally assume the form of a grinding wheel or of tablets, sectors,inserts and other forms which are applied to suitable support plates orpads equipped with coupling means for the connection to a machine bymeans of which the treatment is carried out, or they are directlyconnected to the machine in any other known way. Among the abrasivematerials those belonging to the family of the aluminum oxides, such ascorundum, are commonly used, the silica based compounds such as siliconcarbide, as well as the so-called “superabrasives”, that is natural orsynthetic diamond powder and the synthetic nitrides, for example CBN(cubic boron nitride), essentially used in the field of the treatmentsof metal surfaces.

Up to now, for the treatment of the surfaces of stone-like materials,such as marble, granite and stones in general, or of ceramic materials,such as ceramics, gres and derivatives, abrasive compositions have beenused which comprise an abrasive material, for example diamondiferousmaterial or silicon carbide, agglomerated with a mineral binder, inparticular a magnesian binder possibly with the addition of a phenolicresin, or with resinous binder generally consisting of epoxy or phenolicresins.

The abrasive tools using resinous materials as a binder present severaladvantages with respect to those using magnesian materials, the mostimportant of which is the greater effectiveness of use, that is thegreater speed of performing the treatment on surfaces of the samecharacteristics. They present, on the other hand, a lower supportstiffness of the abrasive particles, what involves the tendency toproduce an increased abrasion on tender materials, and hence they provedto be unsuitable with materials such as granite as regards the “poreclosure”, i.e. the porosity reduction, of the surfaces under treatment.Besides, because of the relative elasticity of the resinous binder,beyond certain operating pressures the abrasive sinks into the bindingmatrix instead of attacking the surface to be treated, therefore, forexample, ceramic surfaces cannot be treated when already installed. Atlast, each surface treatment among those mentioned above, requiresvarious passages with abrasive tools of different grain sizecharacteristics, therefore the treatment times are always relativelylong.

SUMMARY OF THE INVENTION

The main purpose of the present invention is to provide a new abrasivecomposition for the manufacture of tools destined to the execution ofsurface treatments such as roughing, smoothing and polishing on stonematerials and ceramics able to offer higher performances with respect tothe current state of the art, both as the treatment life and thetreatment execution time are concerned.

A particular purpose of the present invention is to provide an abrasivecomposition of the above mentioned kind which makes possible themanufacture of abrasive tools, whose compactness and stiffness can beoptimized as a function of the use conditions, the kind of machine, theoperating pressures, the number of revolutions and other operatingparameters.

A further purpose of the present invention is to provide an abrasivetool for the treatment of surfaces of stone materials and ceramicmaterials characterized by an effectiveness of use considerably higherthan that of the conventional tools.

These purposes are reached with the abrasive composition according tothe present invention, comprising at least a superabrasive and a binderand characterized in that the binder comprises a frit approximatelymelting between 450° C. and 650° C. and a refractory material, saidcomposition further comprising a chemically inert abrasive material forthermally and mechanically protecting said superabrasive.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Preferably the frit is an aluminum silicate based frit containing oxidesof alkaline metals, alkaline-earth metals and boron. Particularlypreferred are the frits of the above-mentioned kind which are lead-freeand have melting temperatures of about 500° C. As refractory materialthere can be used pumice, kaolin, clay, feldspar, magnesia andequivalent materials.

The superabrasive is constituted by diamondiferous material, that issynthetic diamond or natural diamond or mixtures thereof. The grain sizeof the diamondiferous material is typically comprised between 30 and1800 mesh depending on the intended use of the tool (roughing,smoothing, polishing).

As a chemically inert abrasive material there advantageously can be usedcorundum, aluminum oxide, silicon carbide, silica and derivatives. Inthe present description this abrasive material is defined as “chemicallyinert” in view of the fact that, at the operating conditions set forthfor the tool production process, it does not react with the binder, norundergo thermal decomposition. The grain size of the chemically inertabrasive material must generally be lower than that of the superabrasiveas otherwise the latter would turn out screened by the inert one andcould not perform any abrasive function, especially on hard surfaces. Asa matter of fact, the presence of an even limited fraction of abrasivewith a granule size substantially equal to that of the superabrasive hasproven to be useful to mechanically protect the superabrasive fractionand to increase the lifetime of the tool, while, on the other side, asubstantial amount of abrasive with fine granule size has proven to beuseful both to preserve the compactness of the abrasive workpiece and toact as a coating of the superabrasive granule reducing the surfaceexposed to the air and protecting it above all against the hightemperatures that could degrade it. Therefore, advantageously, thecomposition according to the present invention contains two fractions ofabrasive material of different grain size, in particular a fine grainfraction (1.5 to 3μ) and a larger grain fraction, or coarse fraction,with a grain size of the same range as that of the superabrasive. Theproportion of the fine fraction to the coarse fraction is comprisedbetween 2/1 and 5/1 and preferably about 3/1.

In the abrasive composition according to the present invention, thesuperabrasive is present in the conventional per cent concentrationcomprised between 0.2 and 4 Kt/cc, while the binder has the followingweight per cent composition:

frit 50 to 80% refractory 20 to 50%.

Taking into consideration also the inert abrasive and with theabove-mentioned superabrasive concentration, the composition accordingto the present invention is the following:

frit 20 to 80 % wt refractory 10 to 50 % wt inert abrasive 10 to 50 %wt.

The amount of binder depends on the grain size of the diamondiferousmaterial and changes with it; the finer the grain of the diamondiferousmaterial is, the lower is the amount of binder and proportionallygreater is the amount of abrasive.

As a non-limiting example, a particularly preferred abrasive compositionis given herebelow. It contains synthetic diamond powder at aconcentration equal to 1 Kt/cc and the following weight percent of theother components:

frit 40 pumice 30 aluminum oxide 30

The tool made with this composition is particularly suited for carryingout a smoothing treatment.

The frit used in the present invention is a commercial productconsisting of aluminum silicate containing boron and sodium oxides andsmall amounts of potassium and calcium oxides and free from lead. Thegrain size is 10⁴ mesh/cm². The refractory is constituted by whitepumice in powder form of the commercial kind with a grain size lowerthan 400 mesh.

The inert abrasive is essentially constituted by aluminum oxide (over99%), in powder form, with a grain size comprised between 200 mesh, forthe coarse fraction, and 1.5 to 3μ, for the fine fraction.

The manufacture of an abrasive tool by using the composition accordingto the present invention can be carried out as follows. Two mixtures,one containing the diamondiferous powder and the coarse fraction of theinert abrasive and the other one containing the frit, the refractory andthe fine fraction of the inert abrasive, are prepared separately. Thefirst mixture is moistened with a furfural-added phenolic resin to wetthe diamondiferous powder and facilitate the coating of the diamondparticles, while the second one can be added with furfural or equivalentalcohol.

The mixtures are homogenized thoroughly and then mixed together. Theresulting mixture is homogenized and divided into doses of prefixedweight which are cold moulded in the desired shapes. The resultingworkpieces are then heated in a kiln in such a way to cause the meltingof the frit and the successive cooling according to a prefixed thermalcycle, in which the heating is carried out stepwise up to a temperatureat least equal, and generally higher, to the melting temperature of thefrit as a function of the type of refractory used, while the cooling toroom temperature takes place in a natural way in the same kiln, afterthe heat source has been shut off.

As a result of the thermal cycle of heating and cooling the fritvitrifies and mechanically binds the various components, while therefractory keeps under control the consistency of the workpieces duringthe heating to avoid an excessive softening thereof. In other words, therefractory allows the frit to melt and hence vitrify, thereby developingits binding action, but at the same time it assures the consistencynecessary to keep the shape features of the workpieces.

The abrasive tool obtained in this way is for example in the form of acylindrical tablet form and can be applied, for example by glueing, to arigid or semi-rigid disk support or pad in a number adequate to cover acertain portion of its surface. The so equipped support is mounted onthe machine for the execution of the intended treatment by means orconventional coupling means which the support is provided with.Alternately, if the machine is provided with suitable tool carrierspindle, the tools can be applied to the machine directly.

The product which is obtained has a cost higher than that of theabrasive tools using resinous binder but such cost is widely offset bythe greater productivity of the tool which results in markedly lowertreatment time. With the abrasive tool according to the presentinvention it is possible to carry out the treatment of a marble orgranite surface, such as smoothing, through the passage of only one typeof tool, while according to the prior art two of more, different grainsize tools to pass in succession on the surface to be treated arenecessary.

By way of comparison, a test was carried out using a diamond resinoustool of the best commercially available quality and a tool according tothe invention of composition equal to that of the above referred exampleto carry out the smoothing of a 20 m² granite surface which waspreviously submitted to roughing.

The surface was divided in two equal parts which were machined with thetool according to the invention and the commercial one respectively. Inthe first case a 100 mm pad with a 76 mm internal hole was used; ten 12mm diameter cylindrical tablets were applied on an annular surface ofthe disk corresponding to an abrasive surface approximately equal to1130 mm². The nominal grain of the tool was 220. In the second case thesame degree of smoothing and quality of the respective surface portionwas reached using three subsequent pads having nominal grains 60, 120,and 220, respectively. The diameter of the pads was 100 mm with a 78 mminternal hole. On an annular surface of each of them sixteen 13×10×11 mmtrapezoidal tablets were arranged resulting in a total abrasive 25surface approximately equal to 2016 mm². The time necessary for theexecution of the treatment, carried out by the same operator and withthe same machine, turned out to be 15 minutes in the first case and 45minutes in the second one.

The high stiffness of the vitreous matrix embedding the diamond powderallows for the tool according to the present invention to be used torestore damaged ceramic surfaces, for example, scratched or worn bytreading, which would be impossible to treat by means of any otherabrasive tool known in the art.

Variations and/or modifications can be brought to the abrasivecomposition for manufacturing abrasive tools for stone materials,ceramics and the like, according to the present invention withoutdeparting from the scope of the invention as set forth in the attachedclaims.

What is claimed is:
 1. In an abrasive composition for the manufacture oftools for surface treatments on stone and ceramic materials, comprisinga superabrasive and a binder, the improvement wherein said bindercomprises a frit having a melting temperature in the range of 450°-650°C. and a refractory material, said composition also comprising achemically inert abrasive material for thermally and mechanicallyprotecting said superabrasive, wherein said chemically inert abrasivematerial comprises two fractions having different grain sizes, a firstsaid fraction having a grain size substantially equal to that of saidsuperabrasive and a second fraction of finer grain size than that ofsaid superabrasive.
 2. The abrasive composition according to claim 1,wherein said superabrasive is natural or synthetic diamond powder or amixture thereof.
 3. The abrasive composition according to claim 1,wherein said refractory material is selected from the group consistingof pumice, clay, feldspar, and magnesites.
 4. The abrasive compositionaccording to claim 1, wherein the chemically inert abrasive material isselected from the group consisting of aluminum oxide, silicon carbide,and silica.
 5. The abrasive composition according claim 1, wherein saidbinder has the following % wt composition: frit 50 to 80 wt % refractorymaterial 20 to 50 wt %.


6. The abrasive composition according to claim 1, comprising 0.2 to 4Kt/cc of superabrasive and having by the following % wt of components:frit 20 to 80 wt % refractory material 10 to 50 wt % inert abrasive 10to 50 wt %.


7. The abrasive composition according to claim 6, wherein the ratio ofthe finer second fraction to the first fraction of said chemically inertabrasive material is between 2/1 and 5/1.
 8. An abrasive tool having anabrasive surface comprising a composition according to any one of claims1-4 or 3-7.
 9. A method for manufacturing an abrasive tool having acomposition according to any one of claims 1 to 4 or 5 to 7, comprising:preparing a first mixture containing the superabrasive and the firstfraction of the chemically inert abrasive material; preparing a secondmixture containing the frit, the refractory material and the secondfraction of finer grain size of chemically inert abrasive material;wetting the first mixture and the second mixture; blending the twomixtures together to provide a blend thereof; submitting predeterminedquantities of said blend to press molding to obtain pressed workpieces;submitting said workpieces to heating at least to the meltingtemperature of the frit, and subsequently to cooling.
 10. The methodaccording to claim 9, wherein the heating to the melting temperature ofthe frit is carried out in a series of steps in which the temperature isincrementally raised, and is followed by withdrawal of heat whereby theworkpieces are permitted to cool.
 11. The method of claim 9, wherein insaid wetting step, said first mixture is wetted with phenolic resin andfurfural and said second mixture is wetted with furfural.
 12. Theabrasive composition of claim 1, wherein said second fraction of finergrain size is of a size sufficiently small so as to act as a coating ongranules of said superabrasive.
 13. The abrasive composition of claim12, wherein the grain size of (1) the superabrasive granules and (2) thefirst fraction of said chemically inert abrasive material is in therange between 30 and 1800 mesh, and the grain size of said secondfraction of finer grain size is 1.5 to 3μ.